Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to the person skilled in the art and can be based on various effects. Such devices are, for example, cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells ("super-twisted nematic"), SBE cells ("super-birefringence effect") and OMI cells ("optical mode interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have a good chemical and thermal stability and a good stability towards electric fields and electromagnetic radiation. Further, the liquid crystal materials should have a low viscosity and in the cells should give short response times, low threshold potentials and a high contrast. Furthermore, at the usual operating temperatures, that is, in a widest possible range below and above room temperature, they should have a suitable mesophase, for example a nematic or cholesteric mesophase for the aforementioned cells. Since liquid crystals are generally used as mixtures of several components, it is important that the components have a good miscibility with one another. Other properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfil different requirements depending on the type of cell and field of application. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy and an electrical conductivity which is as low as possible.
There has recently been an increased interest in materials having a low optical anisotropy, especially for actively addressed liquid crystal indicators such as MIM applications (metal-isolator-metal) or TFT applications ("thin film transistor") in television sets. However, the known liquid crystalline compounds having a low optical anisotropy frequently produce smectic tendencies in mixtures and for the most part lead to an undesired increase in the threshold potential, the viscosity and/or the response times.